Thyroxine (T4) is a type of thyroid hormone secreted from the thyroid, and it is represented by the structural formula shown below. In general, thyroxine (T4) affects cells throughout the body and accelerates the cellular turnover rate. In addition to thyroxine, triiodothyronine (T3) is known as a thyroid hormone, and thyroxine (T4) accounts for a major share of the thyroid hormones circulating in the blood.

A conjugate of thyroxine and bovine serum albumin (BSA) (hereafter referred to as “T4-BSA”) is used as a polyhapten or the like for preparing an anti-thyroxine antibody. In addition, T4-BSA is utilized in competitive assays of T4 and the like. The present inventors have attained results of competitive assays of T4 and the like indicating that the performance of T4-BSA depends on the degree of purity of T4-BSA instead of the labeling index of T4 for BSA. Since T4-BSA production involves the use of excess T4 derivatives, unreacted T4 derivatives cannot be completely removed via conventional techniques such as dialysis or gel filtration conducted with the use of an aqueous solvent.
A hapten tracer complex comprising a hapten linked directly or via a spacer group to an indicator component and an antibody capable of specifically linking to the indicator component has been known (see JP Patent Publication (Kokai) No. H08-233812 A (1996)) in connection with a technique for eliminating adverse results of analysis caused by unstability and non-specific linkage of a hapten tracer in immunoassay in which hapten is linked to an indicator component. According to the method for producing such complex, an antibody against an indicator component is mixed with hapten linked to an indicator component in an aqueous solution, and an organic solvent (e.g., acetonitrile) is added to the aqueous solution in order to improve the solubility of hapten linked to an indicator component.
However, no technique involving the use of an organic solvent such as acetonitrile for purification of T4-BSA has been known to date.